PLATFORM GAME DESIGNER (PGD)
By Jonathan Cauldwell, for the Sinclair ZX Spectrum 48K upwards, on emulator files.
From the author of the highly acclaimed ARCADE GAME DESIGNER, this toolkit allows you to make your own platform games. It supports LINEAR PROGRESSION games (e.g. Manic Miner style) and EXPLORER games (Jet Set Willy style). Very easy to use, and saves an independent program that runs by itself when you're done.